Baker’s cysts are named after William Morrant Baker, a London surgeon who described them in 1877, but the name does not explain what is happening.
A Baker’s cyst is a pocket of joint fluid that has escaped from the knee and collected in a small sac behind it. The fluid can be drained, but choosing the right treatment depends on confirming the diagnosis and understanding why the knee is producing extra fluid.
This guide explains the causes, what ultrasound shows and when a cyst may need aspiration.
What is a Baker’s cyst?
It forms in a bursa between two structures at the back of the joint. In most adults, this bursa connects to the knee joint through a small opening.
The opening can behave like a one-way valve. When the knee produces extra fluid, some passes backwards into the bursa and cannot return easily. The sac then enlarges and may become a visible lump.
In adults, a Baker’s cyst is therefore usually a sign of another problem inside the knee rather than an isolated condition.
What does a Baker’s cyst feel like?
A Baker’s cyst usually feels like a soft, rounded swelling behind the knee that becomes tighter when the leg is fully straightened.
A visible bulge
The lump is most obvious when standing with the knee straight and often softens when the knee bends.
Tightness or pulling
It can feel as though something is physically blocking the back of the knee.
Reduced bending
Kneeling, squatting or getting into a car may feel restricted.
Clicking in the knee
Clicking usually comes from the underlying joint problem rather than the cyst itself.
Activity-related aching
Symptoms may worsen after exercise or prolonged standing.
Many Baker’s cysts cause no symptoms and are discovered during imaging for another knee problem.
A lump behind the knee should be identified before treatment
A knee ultrasound confirms whether the swelling is fluid-filled, measures it and examines the joint for the likely cause of excess fluid. No GP referral is required.
Knee UltrasoundWhat causes a Baker’s cyst?
A Baker’s cyst forms when the knee produces more fluid than usual. The most common causes are osteoarthritis and a meniscal tear.
A study of 382 knee MRI scans published in MAGMA in 2000 found Baker’s cysts in 38% of patients. Cysts were significantly associated with both the amount of fluid inside the joint and the pattern of meniscal damage.
Inflammatory conditions including rheumatoid arthritis and gout can also increase joint fluid. In children, a cyst may appear without any underlying joint disorder.
How is a Baker’s cyst diagnosed?
Ultrasound is the first-choice test for a swelling behind the knee because it is quick, uses no radiation and can assess the lump during joint movement.
A private knee ultrasound scan shows:
Whether the lump is fluid or solid
This is the first and most important question for any swelling behind the knee.
Its size and neck
The scan measures the cyst and identifies the channel connecting it to the joint.
Whether it is simple or complex
Complex cysts may contain thickened lining, debris or separate compartments and can respond differently to drainage.
Leakage or rupture
Ultrasound can show fluid tracking down into the calf.
Fluid inside the knee
A joint effusion provides evidence about the underlying cause.
What else could a lump behind the knee be?
Several conditions can create a lump or swelling behind the knee. Distinguishing them completely changes the treatment.
| Condition | How it differs |
|---|---|
| Popliteal artery aneurysm | A swollen section of the main artery behind the knee. It may pulse and requires vascular assessment rather than drainage. |
| Deep vein thrombosis | A clot in a deep vein causing a swollen, warm and tender calf without a distinct lump behind the knee. |
| Ganglion or meniscal cyst | A firmer and smaller swelling arising from joint lining or a torn meniscus, often slightly to one side. |
| Lipoma or another soft-tissue lump | A solid rather than fluid-filled lump that needs assessment to establish its nature. |
Swipe sideways to view the complete table.
Is it worth having a Baker’s cyst drained?
Drainage may be worth considering when the swelling is large, painful, restricts knee bending or has ruptured.
The procedure is usually performed as an ultrasound-guided joint aspiration. A needle is placed into the cyst under direct vision, fluid is withdrawn and a corticosteroid injection may follow when clinically appropriate.
Aspiration can relieve symptoms faster than other options. However, roughly half of cysts refill during the following months if the knee continues producing excess fluid.
Treating the knee joint and its underlying condition at the same time usually provides the longest-lasting result.
What is the quickest way to get rid of a Baker’s cyst?
The quickest way to reduce the swelling is ultrasound-guided aspiration, where the fluid is visualised and removed during one appointment.
For a mild cyst, NHS advice includes resting the knee, using an ice pack wrapped in a tea towel for about 10 minutes, elevating the leg on a pillow and taking suitable over-the-counter pain relief.
The cyst often shrinks as the joint settles. Preventing recurrence depends on treating the cause, such as osteoarthritis, a meniscal tear or inflammatory arthritis.
What should you avoid with a Baker’s cyst?
Avoid deep knee flexion because it compresses the cyst and raises the pressure inside it.
- Reduce deep squats and lunges.
- Avoid prolonged kneeling.
- Do not sit for long periods with the knees tightly tucked underneath.
- Never squeeze or attempt to pop the cyst yourself. It lies close to important nerves and blood vessels.
For sleep, many people are comfortable on their back with a pillow under the knee so it remains slightly bent, or on their side with a pillow between the knees.
What if a Baker’s cyst bursts?
A ruptured Baker’s cyst causes sudden calf pain and swelling as fluid tracks downwards. Bruising can appear around the ankle several days later.
This requires prompt assessment because the symptoms can closely resemble deep vein thrombosis. Ultrasound can distinguish the two. A DVT Health Check includes Doppler imaging and a blood test when a clot needs excluding.
What type of doctor should you see for a Baker’s cyst?
Start with a GP or MSK clinician who can examine the knee and arrange imaging. A scan can also be booked directly when there is already a visible lump.
An orthopaedic opinion is useful if the cyst repeatedly returns, the knee locks or gives way, or the underlying joint damage may require treatment. A consultant orthopaedic assessment is most useful when the ultrasound has already been completed.
Book a knee ultrasound at London Private Ultrasound
If you have a lump behind the knee and want to identify it before deciding on treatment, ultrasound can answer that in one appointment.
Experienced since 2012
London Private Ultrasound has provided diagnostic ultrasound services since 2012 and operates within a CQC-registered pathway.
Dynamic assessment
An MSK specialist scans the cyst and knee joint while bending and straightening the leg to see how the swelling behaves.
Findings explained immediately
The clinician explains the images during the appointment and supplies a written report.
Drainage in the same clinic
If aspiration is appropriate, it can be arranged through the Joint Pain Clinic with the same team.
Convenient locations and hours
Appointments are available seven days a week, 9am to 7pm, at 27 Welbeck Street near Bond Street and Oxford Circus and at 54–56 Victoria Street in St Albans.
This service is for adults aged 18 and over. No GP referral is required. Call 020 7101 3377 or contact the clinic via WhatsApp.
Book Knee UltrasoundRelated musculoskeletal services
Knee ultrasound
Assessment of a Baker’s cyst, joint fluid, accessible meniscal margins, tendons, ligaments and bursae.
Knee UltrasoundUltrasound-guided aspiration
Image-guided drainage of suitable cyst or joint fluid, with treatment planning for the underlying cause.
Joint AspirationDVT Health Check
Doppler assessment and blood testing when calf swelling could be caused by a blood clot.
DVT Health CheckOrthopaedic assessment
Specialist review for recurrent cysts, locking, instability or underlying joint damage.
Orthopaedic AssessmentJoint Pain Clinic
Integrated assessment, aspiration, image-guided treatments and rehabilitation pathways.
Joint Pain ClinicOne-Stop Orthopaedic MSK Clinic
Coordinated specialist assessment, imaging and treatment planning.
One-Stop MSK ClinicFrequently asked questions
Will a Baker’s cyst go away on its own?
It can, particularly when the knee condition producing extra fluid settles. Cysts related to ongoing osteoarthritis or an untreated meniscal tear are more likely to persist or return.
How long does a Baker’s cyst last?
It can last from several weeks to years. The timescale depends more on the underlying knee problem than the cyst itself.
What happens if a Baker’s cyst is left untreated?
Most remain stable or fluctuate with the knee. A large cyst may cause calf tightness or press on nearby structures, and occasionally it ruptures.
Is a Baker’s cyst treated on the NHS?
Yes. NHS management usually begins with self-care and treatment of the underlying knee condition. Drainage or surgery is reserved for cysts causing significant symptoms, and imaging waits vary locally.
Can I exercise with a Baker’s cyst?
Yes, using activities that avoid deep knee bending. Swimming, flat walking and cycling with a raised saddle are often comfortable, while deep squats and heavy leg work may aggravate symptoms.
Does a knee support help a Baker’s cyst?
It can reduce discomfort by supporting the joint and limiting deep bending. It treats symptoms rather than the cause and works best alongside strengthening and treatment of the underlying knee problem.
How much does it cost to have a Baker’s cyst drained?
Cost depends on whether the appointment includes diagnostic imaging, corticosteroid injection and laboratory analysis. Current fees are listed on the ultrasound-guided joint aspiration service page.
Does a knee ultrasound hurt, and do I need a referral?
Ultrasound is painless and uses no radiation. No GP referral is required, and a written report is provided for your GP, physiotherapist or surgeon.
Clinical guidance and review
This information reflects NHS self-care guidance, relevant published evidence and ultrasound practice aligned with professional BMUS standards. Diagnosis and treatment should be individualised after clinical assessment.